1. Field of the Invention
This invention relates to a radar detector for detecting a radar wave transmitted by a "police radar gun" which is a radar apparatus used by a police for automobile speed traps.
2. Description of the Prior Art
A police radar gun is an apparatus which transmits a radar wave of a frequency ranging from microwave to millimeter wave frequencies toward a moving car and detects the frequency shift of the reflected wave. The speed of the car is determined via doppler effect. A radar detector, which is placed in an automobile to detect the existence of such police radar gun operating in the vicinity, is designed to capture those radar waves belonging to the frequency bands for the police radar gun.
However, the detector may detect any radar wave as long as its frequency is within the detection target range even if the radar wave is not from the police radar gun. For further discussions on this, a representative configuration example of the prior art radar detector is shown in FIG. 1 and a particular example of the relationship among the frequencies in FIG. 2.
Three radar frequency bands shown below and in FIG. 2 are utilized by police radar guns in the United States of America.
X-band: 10.52.+-.0.025 GHz PA1 K-band: 24.15.+-.0.1 GHz PA1 Ka-band: 34.7.+-.0.5 GHz
Many of the conventionally available radar detectors are designed so that they can detect all radar waves having frequencies in these three bands. In general, the frequency of a wave that is termed a microwave does not exceed 30 GHz and the frequency region exceeding that frequency is designated as a millimeter wave region. Therefore, the Ka-band mentioned above must be included in the millimeter wave region for strict discussions. In the following descriptions, however, the radar waves of the three bands are all referred to as being microwaves for convenience.
Referring now to the configuration of the prior art radar detector shown in FIG. 1, the detector is mainly a receiving system based on a heterodyne method, particularly the double heterodyne method, wherein a radar wave received by an antenna horn 1 is mixed with a first local oscillation signal at a first mixer 2 to "be beaten down" or converted to a lower frequency. The frequency of the beat down signal output from the first mixer 2 is set to around 1 GHz for the input radar waves of any above-mentioned band. Therefore, the signal frequencies of a first local oscillator fed to the first mixer 2 are inevitably defined, and the first local oscillator must oscillate at different frequencies in order to detect the radar waves in each of the respective bands, because each band occupies a respectively different frequency region. In the system of FIG. 1, two local oscillators 6 and 7 are used as the first local oscillator which are switched in a programmed sequence as controlled by a signal processing circuit 10 to operate in turn.
The local oscillator 6 is provided for the X- and K-band radar waves and its fundamental oscillation frequency f.sub.X/K is chosen to be 11.5 GHz for example. When the radar waves in the X-band are to be detected, the output signal of oscillator 6 (i.e. the fundamental oscillation frequency f.sub.X/K (=11.5 GHz) is applied to the first mixer 2 and thus the input signals of the X-band are beaten down to 0.975 GHz with respect to a center frequency.
For the detection of the K-band radar waves, the 23 GHz (f.sub.X/K .times.2=11.5 GHz.times.2) second harmonic wave derived from the local oscillator 6's output signal of the fundamental oscillation frequency f.sub.X/K is advantageously exploited to beat down the K-band input signals to 1.15 GHz with respect to the center frequency. Therefore, instead of providing two exclusive first local oscillators for X-band and K-bands respectively, the fundamental oscillation signal and its second harmonic signal are both utilized so that the circuit is simplified and is reduced in size and in cost.
Another local oscillator 7, which operates according to switching instructions from the signal processing circuit 10, is used only when the Ka-band input signals are to be detected. Its fundamental oscillation frequency f.sub.Ka is chosen to be 16.6 GHz for example to provide the 33.3 GHz second harmonic signal, which is obtained by advantageously utilizing the non-linearity of the oscillator's transistor (not shown) used therein. This 33.3 GHz are signal is applied to the first mixer 2 as the substantial first local oscillation signal to be mixed with the Ka-band signals (center frequency: 34.7 GHz). As a result, the beat down signals of the Ka-band radar waves go out from the first mixer 2 with their center frequency at 1.5 GHz.
In FIG. 2, solid lines indicate the fundamental oscillation frequencies f.sub.X/K and f.sub.Ka of the local oscillator 6 and 7 respectively and dotted lines their respective harmonic frequencies.
The approximately 1 GHz first intermediate frequency signal, which is provided by a first IF amplifier 3, is further converted down at a second mixer 4 and then goes into a detector 9 through a second IF amplifier 5. The frequency of a local oscillation signal applied to the second mixer 4 sweeps in predetermined different ranges depending on the frequency band to be detected. These ranges are chosen so as to make the frequencies of the detected signals output from the second IF amplifier 5 to always be on the order of several tens of MHz generally for the radar waves of any frequency band mentioned above. A second local oscillator 8, which applies the oscillation signal to the second mixer 4, is formed via a voltage controlled oscillator (VCO) for example and causes it oscillation frequency to sweep in accordance with the sweep voltage signal from the signal processing circuit 10.
In a particular time sequence, the signal processing circuit 10 first puts the heterodyne receiving system comprising the circuit elements 1 through 9 into operation in an X- and K-band receiving mode. In particular, the signal processing circuit 10 activates the local oscillator 6 for the X- and K-bands to apply the fundamental oscillation signal of 11.5 GHz and the second harmonic signal of 23 GHz to the first mixer 2. The signal processing circuit 10 also instructs the second local oscillator 8 to let its oscillation frequency sweep from 0.95 GHz to 1.25 GHz. After a predetermined time period for the frequency sweeping, the signal processing circuit 10 changes the receiving mode to a Ka-band receiving mode, thereby actuating the local oscillator 7 for the Ka-band instead of the local oscillator 6 for the X- and K-bands and letting the second local oscillator 8 output a signal which sweeps in frequency from 0.95 to 2 GHz. On completion of this sweeping operation, the signal processing circuit 10 sets the heterodyne receiving system to the X- and K-band receiving mode again and thereafter repeats these operations in sequence. Note that the lower limit of the frequency sweeping range of the signal provided by the second local oscillator 8 is, for the simplicity of the design procedure, generally set to 0.95 GHz in the Ka-band receiving mode as well as in the X- and K-band receiving mode, although in the Ka-band receiving mode the limit frequency 1 GHz is suggested to be sufficient according to FIG. 2.
The detector 9 can be designed as a FM discriminator; in any of the receiving modes specified by the signal processing circuit 10, when the aforementioned signal having a frequency on the order of several tens of MHz has appeared at the output of the second IF amplifier 5, the detector 9 informs the signal processing circuit 10 of the appearance of the signal.
The signal processing circuit 10 then activates a warning system (not shown) through an appropriate I/O interface 11. For the warning system, a sound indicator apparatus using a buzzer or the like and/or a light indicator apparatus using a photodiode or the like may be utilized. Most of the recent radar detectors are equipped with both of these apparatus. With respect to the sound indication, many such radar detectors have functions for turning the sound indication on or off depending on the user's switching operation, adjusting the volume, and so forth. Note here that these functions can also be adopted in the radar detector in accordance with the present invention, which will be described later.
The prior art radar detector shown in FIG. 1, which is a multi-band radar detector capable of detecting the radar waves of any of the three bands, X- and K-, and Ka-bands, may erroneously detect the radar waves which are not exactly from the police radar gun if the waves are of the detection target frequency bands as is understood according to the operation principle described above. In some situations, the radar detector attached to a car is sometimes triggered even though there is no police radar gun in operation in the vicinity. Once the detector raises a warning, the driver reflexively slows down the car speed even if driving safely at speeds within the regulatory limits. Such false warnings, which greatly deteriorate the driving environment, are not desirable and should be reduced as much as possible.
The erroneous detection often arises from the external leakage of the local oscillation signals of other radar detectors as well as by radar apparatus, for example, adopted for automatic opening and closing doors of stores and other such equipment. As described above according to FIGS. 1 and 2, the local oscillators in the radar detectors of this type also generate, owing to their non-linearity, harmonic waves having frequencies which are the integer multiplies of the fundamental oscillation frequencies. The oscillation power of the harmonic waves are partly radiated into the air through the antenna horn 1, even though it is intended only for receiving waves. Therefore, the radar detector operating at the frequencies shown in FIG. 2 radiates, as leaked local oscillation waves, not only the fundamental oscillation wave of the frequency f.sub.X/K =11.5 GHz generated by the local oscillator 6 but the harmonic waves whose frequencies are two, three, four, or more times the fundamental oscillation frequency.
As is evident from FIG. 2, among the harmonic waves, the 34.5 GHz third harmonic wave of the fundamental oscillation signal for the X- and K-band signals is at a frequency within the Ka-band and so, may cause an erroneous detection of a radar signal detection, since signals of frequencies belonging to the Ka-band are the target of detection. In fact, when two cars provided with radar detectors of a similar type pass each other, the radar detectors in both cars will most likely be triggered erroneously.